Phenol is known as an intermediate widely used for the synthesis of synthetic resins, surfactants, medicines and the like. Various processes are known as methods of preparing such phenol as referred to above, however, a method which has been mainly adopted therefor is a so-called cumene process wherein phenol and acetone are prepared by oxidizing cumene with molecular oxygen to cumene hydroperoxide which is then cleavaged with acid to phenol and acetone.
However, where phenol is prepared by the cumene process which comprises oxidizing cumene with molecular oxygen, it is inevitable that dimethyl phenyl carbinol, acetophenone, .alpha.-methylstyrene, .alpha.-methylstyrene dimer, cumyl phenol and the like are formed as by-products in the course of oxidation reaction and consequently the resulting reaction mixture contains, in addition to phenol and acetone, such by-products as mentioned above. Accordingly, a residue (hereinafter sometimes called a phenol distillation residue), which is obtained after the separation by distillation of cumene, phenol and acetone from the above-mentioned resulting reaction mixture, contains such by-products as dimethylphenyl carbinol, acetophenone, .alpha.-methylstyrene, .alpha.-methylstyrene dimer, etc. Recovery of useful substances by converting these by-products is a matter of importance from the standpoint of reduction of the production costs of phenol by the cumene process.
Heretofore, the recovery of such useful substances as cumene, .alpha.-methylstyrene and phenol from this phenol distillation residue has been practiced by pyrolytically decomposing the phenol distillation residue, followed by distillation or the like. In the prior art process which comprises pyrolytically decomposing the phenol distillation residue, however, there were involved such problems that it takes a long period of time to complete the decomposition and, moreover, recoveries of cumene, .alpha.-methylstyrene and phenol are low.
For the purpose of solving such problems as mentioned above, Japanese Patent Publication No. 36892/1984 discloses processes for recovering useful substances such as cumene, .alpha.-methylstyrene and phenol from the phenol distillation residue resulting from a process for preparing phenol by the cumene process wherein cumene, phenol and acetone are separated by distillation, which processes are characterized in that said phenol distillation residue is pyrolytically decomposed in the presence of alumina catalysts such as .alpha.-alumina and the like or silica-alumina catalysts such as silica alumina, acid clay, synthetic zeolite and the like, or in the presence of these catalysts mentioned above and acids.
According to examples concretely disclosed in Japanese patent publication No. 36892/1984 cited above, however, it has been found that the processes embodied in the examples involve such problems that because the silica-alumina catalysts used have a large particle diameter such as 110-500 .mu.m and the Al.sub.2 O.sub.3 :SiO.sub.2 ratio of from 10:90 to 50:50 and, moreover, the reaction temperature employed is elevated finally up to 340.degree.-345.degree. C., the recovery of .alpha.-methylstyrene from dimethylphenyl carbinol, .alpha.-methylstyrene dimer, o-cumyl phenol and p-cumyl phenol is as high as 70-100%, whereas the recovery of phenol therefrom is markedly low as low as 3-30%.